Ink jet printing, as one type of liquid droplet ejection, has become recognized as a prominent contender in the digitally controlled, electronic printing arena for advantages such as its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be generally categorized by technology, as either drop on demand ink jet or continuous ink jet devices.
The first technology, drop-on-demand ink jet printing, typically provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of an ink droplet that crosses the space between the print head and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. With thermal actuators, a heater, located at a convenient location, heats the ink causing a quantity of ink to change phase, forming a gaseous steam bubble. This increases the internal ink pressure sufficiently for an ink droplet to be expelled. The bubble then collapses as the heating element cools, and the resulting vacuum draws fluid from a reservoir to replace ink that was ejected from the nozzle.
Piezoelectric actuators, such as that disclosed in U.S. Pat. No. 5,224,843, issued to van Lintel, on Jul. 6, 1993, have a piezoelectric crystal in an ink fluid channel that flexes when an electric current flows through it, forcing an ink droplet out of a nozzle. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.
In U.S. Pat. No. 4,914,522, issued to Duffield et al. on Apr. 3, 1990, a drop-on-demand ink jet printer utilizes air pressure to produce a desired color density in a printed image. Ink in a reservoir travels through a conduit and forms a meniscus at an end of an ink nozzle. An air nozzle, positioned so that a stream of air flows across the meniscus at the end of the nozzle, causes the ink to be extracted from the nozzle and atomized into a fine spray. The stream of air is applied for controllable time periods at a constant pressure through a conduit to a control valve. The ink dot size on the image remains constant while the desired color density of the ink dot is varied depending on the pulse width of the air stream.
The second technology, commonly referred to as “continuous stream” or “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes. When no print is desired, the ink droplets are directed into an ink-capturing mechanism (often referred to as catcher, interceptor, or gutter). When print is desired, the ink droplets are directed to strike a print medium.
U.S. Pat. No. 1,941,001, issued to Hansell on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al. on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This early technique is known as binary deflection continuous ink jet.
Later developments for continuous flow ink jet improved both the method of drop formation and methods for drop deflection. For example, U.S. Pat. No. 3,709,432, issued to Robertson on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitude stimulations resulting in longer filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a receiving member.
U.S. Pat. No. 6,079,821, issued to Chwalek et al. on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and to deflect those ink droplets. A print head includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a receiving medium, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher.
U.S. Pat. No. 6,588,888, issued to Jeanmaire et al. on Jul. 8, 2003, discloses a continuous ink jet printer capable of forming droplets of different size and having a droplet deflector system for providing a variable droplet deflection for printing and non-printing droplets.
One well known problem with any type of inkjet printer, whether drop-on-demand or continuous flow, relates to precision of dot positioning. In a printhead with an array of tiny ink nozzles, individual nozzles can differ slightly in fabrication and performance. Slight nozzle differences within tolerance may, for example, affect the trajectory direction of droplets ejected from a printhead, either in the direction in which the print head is scanned (typically referred to as the fast scan direction) or in the direction in which the receiving medium is periodically stepped (typically referred to as the slow scan direction, usually orthogonal to the fast scan direction). Slight errors in trajectory result in corresponding placement errors for printed drops. Another possible error source for dot placement is response time, where each nozzle does not emit its droplet of printing ink with precisely the same timing. This can cause displacement errors in the scan direction. As a result of such fabrication differences and timing response, dot positioning on the print medium may vary slightly, pixel to pixel. For the most part, these minor differences result in placement errors no larger than some fraction of a pixel dimension. For example, where pixels may be placed 30 microns apart, center-to-center, typical errors in dot placement are on the order of 2 microns or larger.
Under some conditions, small placement errors within this sub-pixel range of dimensions may be imperceptible in an output print. However, as is well known in the imaging arts, undesirable banding effects can be the result of a recurring pixel positioning error due to the printhead or its support mechanism. Such banding is typically most noticeable in areas of text or areas of generally uniform color, for example, and can severely compromise the image quality of output prints. One solution used to compensate for banding effects is the use of multiple banding passes, repeated over the same area of the printed medium. This enables a printhead to correct for known banding errors, but requires a more complex printing pattern, requires a more complex and accurate medium transport mechanism, and takes considerably more time per print. Under worst-case conditions, correction for band effects can result in significant loss of productivity, even as high as 10× by some estimates.
Typically, users of inkjet printers are forced to accept a level of relative inaccuracy in dot placement. It can readily be appreciated that it would be desirable to correct slight droplet placement errors by controlling the operation of individual nozzles of a print head, thus obviating the need for multiple banding passes. Proposed solutions for adjusting dot placement with ink jet printing apparatus of various types include the following:                U.S. Pat. No. 6,457,797 (Van Der Meijs et al.) discloses using timing changes to offset the effects of print head temperature changes on relative dot placement for a complete nozzle array in a drop-on-demand type ink jet printer;        U.S. Pat. No. 4,956,648 (Hongo) also discloses manipulating timing intervals for correcting slow and fast scan dot placement in a drop-on-demand type ink jet printer, segmenting the unit dot pitch time interval into suitable sub-intervals;        U.S. Pat. No. 6,536,873 (Lee et al.) discloses bidirectional droplet placement control in a drop-on-demand type ink jet printer, using heater elements to alter the shape of an ink meniscus after the ink is expelled from a nozzle;        U.S. Pat. No. 4,347,521 (Teumer) discloses a print head employing a complex set of electrodes for droplet deflection in a continuous ink jet apparatus;        U.S. Pat. No. 4,384,296 (Torpey) similarly discloses a continuous ink jet print head having a complex arrangement of electrodes about each individual print nozzle for providing multiple print droplets from each individual ink jet nozzle;        U.S. Pat. No. 6,367,909 (Lean) discloses a continuous ink jet printing apparatus employing an arrangement of counter electrodes within a printing drum for correcting drop placement;        U.S. Pat. No. 6,517,197 (Hawkins et al.) discloses an apparatus and method for corrective drop steering in the slow scan direction for a continuous ink jet apparatus using a slow-scan droplet steering mechanism that employs a split heater element;        U.S. Pat. No. 6,491,362 (Jeanmaire) discloses an apparatus and method for varying print drop size in a continuous ink jet printer to allow a variable amount of droplet deflection in the fast scan direction with multiple droplets per pixel;        U.S. Pat. No. 6,217,163 (Anagnostopoulos et al.) discloses a continuous ink jet apparatus and method that provides ink filament steering using a segmented heater to compensate for drop placement inaccuracy;        U.S. Pat. No. 6,213,595 (Anagnostopoulos et al.) discloses a continuous ink jet apparatus and method that provides ink filament steering at an angle offset from normal using power-adjustable segmented heaters;        U.S. Pat. No. 6,508,543 (Hawkins et al.) discloses a continuous ink jet print head capable of displacing printing droplets at a slight angular displacement relative to the length of the nozzle array, using a positive or negative air pressure;        U.S. Pat. No. 6,572,222 (Hawkins et al.) similarly discloses use of variable air pressure for deflecting groups of droplets to correct placement in the fast scan direction;        U.S. Patent Application Ser. No. 2003/0174190 (Jeanmaire) discloses improved measurement and fast scan correction for a continuous ink jet printer using air flow and variable droplet volume; and,        U.S. Pat. No. 4,275,401 (Burnett et al.) discloses deflection of continuous ink jet print droplets in either the fast or slow scan direction using an arrangement of charging electrodes.        
As the above listing shows, there have been numerous proposed solutions for adjusting print droplet trajectory in both drop-on-demand and continuous inkjet printing apparatus. In general, these solutions include approaches such as altering the timing of dot formation or providing a steering mechanism that is external to the fluid chamber from which droplets are ejected, or applying gas pressure, heat, or electrostatic charge to ejected fluid, for example. While each of these solutions may provide suitable steering performance, there is room for improvement, particularly for drop-on-demand print heads. There are inherent difficulties in controlling fluid meniscus formation with drop-on-demand devices and, related to these difficulties, some degree of inherent inaccuracy in droplet steering. In particular, it can be appreciated that there would be advantages to a droplet steering solution that is internal to the fluid chamber of the ejecting mechanism itself. This type of solution could be produced at a favorable cost and enjoy improved robustness, because it would not require an external steering mechanism that must be properly aligned for cooperation with each individual ejecting nozzle.
One approach using droplet steering internal to the fluid chamber is disclosed in Japanese Patent Abstract Publication 2002-240287 by Eguchi Takeo et al. The Takeo et al. publication discloses a drop-on-demand printhead nozzle equipped with a plurality of heaters, wherein one or more heaters is energized for ejecting a liquid drop with a desired trajectory. In the Takeo et al. device, any one of the internal heaters is individually capable of providing sufficient threshold energy for fluid droplet formation. Droplet steering is then effected by asymetrically modulating the energy supplied by one or more heaters. While this solution may provide some measure of droplet trajectory modulation, the Takeo et al. apparatus energizes the same heaters for both droplet formation and droplet steering. Due to inherent instability in the process of forming and releasing a droplet at each nozzle, fine-tuning, by which an individual droplet trajectory can be corrected to within a few microns or less, can be difficult to achieve using such an approach.
Thus, it can be seen that there is a need for an improved apparatus and method for controlling the trajectory of ejected droplets with both drop-on-demand and continuous flow print heads.